SceneGraph: added AbstractObject::transformationMatrix().

Complement for Object::transformation(), usable from features which
don't want/need to upcast AbstractObject pointer.
Math: Vector::lerp() can take `t` of arbitrary type.

Usable when interpolating e.g. colors in denormalized integral form, as
`t` could be only `0` or `1` in that case, which is pretty unusable.